This invention relates to high energy ignition circuits for igniting a fuel. Such ignitor circuits are frequently employed in power plants, steam generation boilers, jet engine ignition and other combustion engineering applications. To ensure that the fuel ignition is achieved, these applications require that the ignition circuit provide a spark at a spark rate and energy level sufficient ignition of the fuel. An uneven spark rate may prevent reliable ignition. In severe cases, uneven spark rates cannot obtain ignition of the fuel.
FIG. 1 shows a block diagram of a conventional high energy ignitor circuit. The typical high energy ignition circuit consists of one or more capacitors 10 that are charged from a direct current (DC) power source 12 through a series resistor 14. Capacitor(s) lo is coupled to a spark gap tube 20 which is in turn coupled to a spark plug 21. Capacitor 10 is charged exponentially by DC power source 12 until the breakdown voltage of spark gap 20 is obtained. At the breakdown voltage threshold of the spark gap, energy is transported across the electrodes and fires the surface gap spark plug.
Conventional ignitor circuits may also contain a monitoring circuit 22 to verify that plug 21 is being fired. The monitoring circuit allows for the prevention of a fuel valve opening with a disfunctioning ignitor circuit. Unignited fuel presents a serious safety hazard; proof of ignition energy is often required by safety codes.
FIG. 2 shows a cut away view of a conventional spark gap tube 20 used in the typical ignitor system described above. Spark gap tube 20 consists of two electrodes, 23 and 24 located within a housing 26 formed of glass or ceramic material. The interior 28 of housing 26 is a controlled environment of an inert gas. Electrodes 23 and 24 are separated a given distance, forming a gap 30. At a predetermined potential energy, current passes from electrode 23 to electrode 24 as arc 32.
FIG. 3 contains a graph of gap breakdown voltage versus pressure. As is evident from curve 34, the breakdown voltage for a given gap is not constant but varies with pressure, and thus temperature. Because interior 28 of spark gap tube 20 is not vacuous, the breakdown voltage is finite and varies across a range of operating conditions.
Use of a spark gap tube therefore imposes several limitations on the typical high energy ignitor system described above. First, the capacitor discharge voltage is not independently variable, but is fixed by the breakdown voltage of the spark gap tube. The breakdown voltage of the spark gap, however, is not constant and varies with pressure and temperature as well as cycles of use. The discharge energy of capacitor 10, therefore, must also vary as a function of temperature, contact erosion, gas 28 contamination, and self-heating, independently of the energy requirements for fuel ignition. To ensure sparking, capacitor 10 must be sized large enough to provide sufficient energy for all environmental conditions and is thus sized to account for variations in the operating characteristics of spark gap tube 20. This fact imposes a design constraint which makes it difficult to optimize capacitor 10.
In addition to governing the size of capacitor 10, the breakdown voltage of spark gap tube 20 fixes the discharge energy of capacitor 10 and prevents independent variation of the discharge energy to respond to changing uses or conditions. For example, the voltage required to fire a spark plug increases in proportion to the number of firings to which the plug has been subjected. To account for these variations over the life of the plug, the gap breakdown voltage and capacitor discharge energy of most ignitor circuits are set at an energy level sufficient to fire a high cycle plug. This same energy level is applied to all plugs no matter how many firing cycles the plug has actually endured and thus in disregard of the true energy requirements of the plug. This practice increases system energy costs and may accelerate plug wear.
A second limitation of the typical ignitor system relates to control of the spark rate. The capacitor of the typical system will fire whenever the breakdown voltage of the gap is reached. Therefore, the spark rate will be governed by the time necessary to charge the capacitor to the breakdown voltage level. The firing rate of the circuit will thus vary according to changes in the spark gap breakdown voltage. Furthermore, line voltage variations can cause fluctuations in the time necessary for power supply 12 to charge capacitor 10 to the breakdown voltage gap of 30 of spark gas tube 20. These fluctuations in the output of DC power source 12 will affect the spark rate unless expensive voltage regulators are used to regulate DC power source 12.
A third limitation of the typical circuit stems from the fact that when the spark plug fires, the length of time that current flows through the spark plug circuit is very short, typically 50 microseconds. This short time period makes it impractical to directly operate an indicating device such as a relay that would require current for a number of milliseconds. Furthermore, the very low impedance of the spark plug, typically 20 milliohms, would cause the insertion of an indicator device in series with the spark plug to rob most of the energy and reduce the circuit efficiency to an unacceptable level. Conventional designs commonly actuate an indicating device via the voltage drop across timing resistor 14 when the full wave rectified, unfiltered, output of high voltage supply 12 charges energy storage capacitor 10. This circuit suffers the disadvantage that a shorted capacitor 10 will provide the same effect as would the periodic discharge of energy storage capacitor 10 through spark plug 21 via spark gap tube 20, thus providing a false and dangerous indicating of current through the spark plug. Additionally, the relay used as the indicator device is a special type of relay with a delayed drop out characteristic. This special type of relay is less reliable and more expensive than a simple relay.